We briefly review some of the approaches which have been used to study the distributions of defect properties in amorphous silica and focus mainly on the implementation of the embedded cluster method. We illustrate some of the results obtained using this method and discuss the remaining problems using the example of oxygen vacancy defects in amorphous SiO 2 . The neutral vacancies are characterized by a wide distribution of formation energies and structural parameters. Our modelling predicts the two major structural kinds of positively charged vacancies (E centres): dimers and dangling bond centres. The local structure for both kinds of centres depends on the medium range structure of the surrounding amorphous network. We found that the majority of the dangling bond centres are unpuckered. The optical spectra and electron paramagnetic resonance parameters calculated for all defects are in good agreement with experimental data. The structural criteria which favour the formation of different kinds of centres in the original amorphous structure are formulated in terms of the average Si-O distance of oxygen ion with its two neighbouring silicon ions.
We present the results of ab initio calculations of several intrinsic and oxygen-containing defects in CaF 2 including an F center, a substitutional O − ion, an O 2−-vacancy dipole, and F A (O 2−) and F + 2A (O 2−) centers. The calculations have been performed using a hybrid density functional and an embedded cluster method. The calculated optical absorption (OA) spectra and magnetic properties are in a very good agreement with available experimental data. It is suggested that isolated substitutional O − ions induce an OA band in the vacuum ultraviolet region at about 7 eV. The nature of the OA bands associated with O 2−-vacancy dipoles and other, more complex, defects is clarified and corresponding luminescence mechanisms are discussed.
We present the results of atomistic and ab initio simulation of several different tilt grain boundaries (GB) in silicon. The boundary structures obtained with genetic algorithm turned out to have no coordination defects, i.e. all silicon atoms restored their tetrahedral coordination during the structure optimisation. That concerns previously known symmetric Σ5 (130), Σ3 (211) and Σ29 (520) boundaries as well as previously unknown asymmetric Σ9 (255)/(211), Σ3 (255)/(211) and Σ13 (790)/(3 11 0) structures. We have performed an extensive study of defect segregation on the boundaries, including neutral vacancy and carbon, phosphorus and boron impurities. A clear correlation between the segregation energy of the defect and local geometry of the boundary site where the defect is segregated has been revealed. We suggest a simple purely geometric model for evaluation of approximate segregation energies of the listed defects.
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